Parting off tools

[The Mad Hat Man]

Having recently purchased a parting off tool for my lathe, I find that it doesnt seem to like cutting even aluminium! The tool needs a lot of pressure to start cutting and then seems to "grab" and leaves a poor finish. As this is a very thin (circa 1.5mm) blade, I am concerned over it breaking.

Does anyone have any ideas how I can get this tool to work properly? I have tried sharpening the tool, ensuring that it has side clearance etc, but it seems to make no difference. .

 

[porker]

With parting off it's all about the stiffness and rigidity of the tool (and everything else) and keeping the pressure on. Parting off is one of those things that when being too cautious the issues start, which leads to more caution etc. On my Myford I moved to parting off from the rear as I could get a stiffer setup on a dedicated rear parting tool than off the Dickson post on the front.

What sort of parting tool is it? Is it HSS or a carbide tip? Also what lathe do you have? Reason for asking is that HSS can be sharpened sharper than standard carbide tips although carbide tools are now made that better suit smaller (less rigid) lathes. It used to be that carbide was not well suited to small lathes because they relied on more pressure that needed a rigid lathe. For a lot of work I still prefer HSS on a small lathe like a Myford but do have some nice carbide tooling for boring etc. that works well.

Guessing you may already have addressed these issues but
- have as little 'stick out' or overhang of the tool as possible (can be tricky if parting a large diameter)
- lock any gibs down when parting (again for rigidity)
- make sure the parting tool is going in perpendicular to the workpiece
- keep the pressure on and cutting
- use some lube (WD-40 is OK with aluminium)

Take a look at parting off on Youtube - guys like doubleboost in his early videos cover this as well as mrpete222
Some aluminium can be a bit 'sticky'. Worth also trying with a bit of mild as it can cut more cleanly

 

[The Mad Hat Man]

It is a small Warco WM180, which is not the most rigid of lathes. I think it is a HSS blade from memory. It looks like i will just have to be brave .

 

[TFrench]

It is a small Warco WM180, which is not the most rigid of lathes. I think it is a HSS blade from memory. It looks like i will just have to be brave .

Hone it with a diamond plate as well, get it really nice and sharp.

 

[Inspector]

Is the tip of the parting tool the same height as the centre of the part? Lubrication?

Pete

 

[porker]

I forgot to mention centre height. This needs to be spot on as Inspector states

 

[MusicMan]

I agree with almost all of that already said, but usually the recommendation on tool height is to make it a shade (0.1mm) above the centre height if parting from the front. This is because if the lathe is not very stiff, or if the tool is hanging out too far, it can flex and be pulled down and under the centre, especially near the end of the cut.

As porker says, steady pressure, keep your nerve and keep going. For aluminium, WD40 is surprisingly quite a good lubricant/coolant. For steels 3 in 1 is fine, tool sharp but not heavily raked (sloping down away from the horizontal. For plastics nothing, but don't go so hard that it melts, and have a sharper tool with a bigger rake. For brass no lube but an intermediate rake.

Attend to your stiffness, tool height, sharpness and rake every time.

 

[chaoticbob]

It sounds to me that this might be a feed/speed problem. You don't say what diameter work you are parting, or at what speed (rpm). It's not uncommon to get a bit of chatter when starting a cut - you just have to be bold and plunge the tool until it starts biting then adjust your feed to 'keep the tool happy'. Sorry if that's not very helpful -it's a bit of a black art, especially on a light lathe, one of those things which is much easier to demonstrate than describe in words. But with a bit of experimentation with speeds and feeds and taking on board the advice others have given above you'll get there! Your lathe should be well capable of parting with a 1.5mm blade.
Robin.

 

[pcb1962]

Not the cheapest option, but I have had absolutely zero problem parting anything since I started using one of these

 

[hawkeyefxr]

I was always taught the the tool piece should be just below the centre of the work piece. If it's above you will be able to make a cut but you will have to force it (which sounds like you are doing) plus you will never be able complete a parting off down to the little pip.
If you are going make multiple partings use a small stone to touch up the FRONT of the tool, never the top (the rake).
After you have made you parting off look closely at the cutting edge, you will see a tiny build up of the metal you are cutting, this is what you are removing with the stone.
One of my jobs was making 2in (50mm) diameter stainless steel, i did this on automatic feed. I don't recommend this though.

 

[The Mad Hat Man]

So... the parting off tool should be slightly above, exactly centre or slightly below Centre height !

Mine is spot one and I am using cutting fluid. I do like the look of the Glanze type tho. I might have to remortgage the house and get one .

 

[srp]

I agree with the 'just below' centre height - the reason is that if the tool deflects the depth of cut slightly reduces, tending to avoid a dig-in and consequent breakage. If set above centre height any deflection increases the chip size and therefore the load. By 'just below' I'm talking about a few thou of course - easy to gauge if you use the ruler trick to set tool height.
Also important to align the tool at right angles to the lathe centre line accurately - I use a dti. Slower speed than for conventional turning, plenty of cutting oil and a constant steady feed.

 

[AES]

To my great surprise, personally I've always had more problem with boring (big holes) than with parting off on my Chinese mini lathe - definitely NOT the most rigid of machines we'd all agree!

I use a very thin (perhaps 3 mm or less - HSS) tool in a holder from the front as "normal", mounted in a QCTP. I make sure to get the absolute MIN overhang possible, make sure the tool is set EXACTLY at 90 degrees to the bed, is SHARP (with a TINY round over at the very tip - very fine diamond file/lap), and make sure it's set at EXACTLY centre height. I set rpm by ear, not too fast (no rev counter on my lathe), and a "not too aggressive" feed in.

I use lube with nearly everything, WD40 or paraffin on ali, a synthetic cutting fluid with everything else.

To my VERY great surprise (based on all the reading I'd done, and the minimal turning I did during apprenticeship) I expected big problems, but it's all gone very well indeed and I've never had a problem/broken a tool (as above, as opposed to when boring).

One tip I did pick up from one of the Mini Lathe web sites a while back is that using such a thin tool and when parting off a fairly big dia work piece, it's best to start the first cut just on the waste end, then when about halfway through, start a second cut overlapping the first, but this time with the tool set to the part off length required.

Although in cross section the tool is slightly narrower/thinner at the bottom that at the top, when going into a deep slot (which is how a parting off job starts out after all) you can get some binding between the sides of the tool and the work piece, The 2nd cut eliminates this possibility (or has for me so far anyway).

As said, I really couldn't believe just how easy I found parting off to be - really a genuine case of jammy blind beginner's luck.

 

[Robbo3]

One tip I did pick up from one of the Mini Lathe web sites a while back is that using such a thin tool and when parting off a fairly big dia work piece, it's best to start the first cut just on the waste end, then when about halfway through, start a second cut overlapping the first, but this time with the tool set to the part off length required.

Although in cross section the tool is slightly narrower/thinner at the bottom that at the top, when going into a deep slot (which is how a parting off job starts out after all) you can get some binding between the sides of the tool and the work piece, The 2nd cut eliminates this possibility (or has for me so far anyway).

Common practice in wood turning. I wondered if the same should be carried out with metal.

Turners also have the option of fishtailing, waggling the handle from side to side to make a slightly wider cut.

 

[novocaine]

Always told just below centre but thats only to deal with catching in hardening steels. Everything else can be done at centre. With a modern parting off tool (sliding hss or carbide insert) you may need to do as AES says. If you grind your own its less of an issue as you should grind a back clearances.

I posted about a very cheap buy worthy book in another thread yesterday. The amateurs lathe by h. Sparey. 6 quid and for all those with a small lathe it should be considered the bible.
My lathe is a bit big to be considered a home lathe but i still have a copy.

 

[Peri]

I would say :-

Tool at centre height or a hair below centre.
Perpendicular to the job (very important).
Relief passes (as AES stated) when cutting larger stock.
Make sure the tool has a top rake - most new knives don't have this, you'll need to grind one.
A surprisingly high rpm (increasing as the stock diameter decreases - ideally keeping a high surface speed).
Plenty of lubrication.
Don't be scared to push the tool, and keep a steady feed.

FWIW I teach apprentices on lathe/mill/basic CNC at a college (admittedly it's on fairly large machines), and the problems I see most often are being too timid ie low rpm's and low tool feed rates, which just create heat and chatter.

 

[AES]

+1 for novocaine's book tip "The Amateurs' Lathe". Rather old now (some of the lathe tool shapes shown are really old) but no matter, the whole thing, including tool shapes, is still IMO, remarkably relevant. I would say a VERY useful book for just about any lathe user, especially those lathes lacking in rigidity (like my Chinese Mini Lathe).

ALSO: I have a copy of "Using the Small Lathe" by L.C. Mason (publisher MAP Technical Publications, ISBN 0 85344 058 1). First published 1963 (so a little "newer" than the above!) but again IMO a highly valuable read and reference. Just to give a "flavour", Chapter Three "Parting Problems" includes Prelim Checks; Tool Shapes; Setting Up; Back Tool Post; Pipless Parting; Extra Thin Blades.

In other words, highly relevant to this thread!

Highly recommended.

 

[hawkeyefxr]

Wow this is like a highly technical procedure on parting off. I am not knocking any of it just find it fascinating.
One thing i have not seen here is, a parting off tool should not cut on the side faces of the tool bit. (this was from Old Harry that taught me in the toolroom during my apprenticeship many years ago. Great man, RIP)

 

[Peri]

One thing i have not seen here is, a parting off tool should not cut on the side faces of the tool bit

Perpendicular to the job (very important).

If it's cutting on the side, the tool isn't at 90 degrees to the job.